1. Field of the Invention
The invention relates generally to the field of semiconductor processing. More specifically, the invention pertains to a reflector plate used in a semiconductor thermal processing chamber, such as a rapid thermal processing chamber.
2. Background
Rapid thermal processing (RTP) is a process for annealing substrates during semiconductor fabrication. During this process, thermal radiation is used to rapidly heat a substrate in a controlled environment to a maximum temperature of over nine hundred degrees above room temperature. This maximum temperature is maintained for less than one second to several minutes, depending on the process. The substrate is then cooled to room temperature for further processing. The semiconductor fabrication process has several applications of RTP. Such applications include thermal oxidation (a substrate is heated in oxygen or a combination of oxygen and hydrogen which causes the silicon substrate to oxidize to form silicon dioxide); high temperature soak anneal (different gas mixtures such as nitrogen, ammonia, or oxygen are used); low temperature soak anneal (typically to anneal wafers deposited with metals); and spike anneal (primarily used in processes where the substrate needs to be exposed to high temperatures for a very short time). During a spike anneal, the substrate is rapidly heated to a maximum temperature sufficient to activate a dopant and cooled rapidly to end the activation process prior to substantial diffusion of the dopant.
High intensity tungsten or halogen lamps are used as the source of thermal radiation. A reflector plate (as shown in FIG. 2 and described further below) aids in maintaining temperature uniformity as the reflector plate reflects heat radiation emitted from the wafer back toward the wafer.
FIG. 1 shows a side cross-section of an existing reflector plate 27. As shown in FIG. 1, radiation pyrometer light pipes 42 protrude through an opening in the reflector plate 27 so that they have a clear view of the wafer, as best seen in FIG. 2. Existing reflector plates are made from aluminum. The pyrometer light pipes 42 are flush with the aluminum reflector plate 27 face on which there is a reflective coating (not shown) and which faces the wafer. Because the light pipes and reflective coating are exposed to the chamber environment, wafer byproduct material can deposit on both the light pipes and/or the reflective coating, which causes a drift in temperature measurement. This drift can occur quickly and drastically or in small increments over a long period of time. Furthermore, the reflective coating applied to the aluminum reflector plate is complex and difficult to manufacture (costly), has a maximum operating temperature limit of 150° C., and has been prone to peeling under certain process conditions. A quartz plate 60 is placed between the wafer and the reflector plate 27, and the quartz plate 48 rests upon standoffs 64 affixed to the reflector plate 27, leaving a gap 62. The quartz plate 48 helps mitigate some of the problems mentioned above. However, there is still a need to minimize the issues discussed above with respect to existing reflector plates.